This invention relates to a printing method and system whereby the information printed on a substrate, such as paper, is transferred in the form of a latent image or "secure image" which is invisible to the eye and any other usual image detecting device at the time of printing and is revealed only after the substrate is subjected to a subsequent process of image activation. This invention is also interpreted as providing a system whereby the initial process of information printing instantly seals and secures the printed message in a way equivalent to the centuries old process of securing printed information by enclosing a letter in an opaque envelope, without the need of an "envelope". The subsequent process of image activation corresponds to the classical process of "tearing the envelope" to reveal the enclosed message or information.
It is understood of course that over the whole time of the history of printing inks, a search and a fascination for invisible inks has always existed. Many such ink systems have been found, developed and used in a limited way mainly because of the limited accessibility of such invisible inks and delivery systems for the latter.